Cyclic Allowable exhaust spread Temp at GE Frame 6 Turbine

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Faisal F. Al-Masri

It was observed recently that the allowable spread temperature for GE Frame 6 turbine is fluctuating between 100F and 300F. the fluctuation was due to changing the bias value from 100 to 200 constantly. i am after getting the source of this fluctuation.

Although the compressor discharge pressure (CPD) reading is steady, the turbine's 18 exhaust Thermocouples reading has average spread temperatures of (80, 75, 75). the allowable spread temperature keeps in changing from 100 to 300 F.

I would like to know what could cause this fluctuation to happen in the turbine exhaust allowable spread temperature?

The turbine is manufactured by GE and it is Frame 6 model that is controlled by Mark VI eTCSS.
 
You need to look at the .m6b file (the sequencing, or, application code) and see how TTXSPL (the eXhaust Spread Limit).

You should see that the bias is "enabled" on certain types of load changes and other transient conditions, and when the transient condition "settles" the bias is removed over time. Until the next transient condition.
 
Spread programme is basically a combustion monitoring software. The accuracy of the program depends fairly on the condition of your TTXM & CTDA thermocouples. If your machine is on load, the combusiton software is in enabled mode,check L83SPM.

Only upon being enabled, can the software help in determining any deterimental effects if at all to your combusiton hardware. Further to this, the average range of allowable spread would be in the region of TTKSPL6 - TTKSPL7 degree centigrade (refer your site control specificaitons).

An allowable bias is always mentioned with mnemonic TTKSPL8 which will have a delay of around 100 seconds i.e. during any turbulent conditions related to load, frequency & for the bias would automitacally be added to the the existing spread values and the the new value would be retained for around 100 seconds or the time for which turbulence exists (whichever is higher. Upon stabilisation of the unit, the spread limit would revert back in the range mentioned above with a pre-set redution value.
 
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